Tonight isn’t the first time the leading Republican gubernatorial candidates have skirmished, but it’s arguably the most important clash.

That’s because Michigan Attorney General Bill Schuette and Lt. Gov. Brian Calley will share the stage with their Democratic counterparts, who are locked into their own fierce contest.

The debate, held in the Grand Hotel on the sidelines of the Detroit Regional Chamber’s Mackinac Policy Conference, comes as absentee voting begins in a couple weeks’ time.

Casual observers would be mistaken if they thought this was a GOP get-together. Yes, many of the attendees lean Republican, but the annual confab on Mackinac Island is more about big business and influence-peddling than main street.

Democrats always have a strong presence here. In fact, then-Gov. Jennifer Granholm and then-Detroit Mayor Kwame Kilpatrick were fixtures during their overlapping times in office. Even John Conyers, the ex-congressman who was hardly a free marketeer, used to hold court.

For Republican politicians, who flock here in droves, the Mackinac Policy Conference is a junket as much as anything else. This despite the fact that the out-state voters at the base of the GOP’s unified control of state government aren’t in-sync with Detroit interests.

If you’re Calley or Schuette this is where you want to be — if only because the press corps, both Lansing and Detroit, decamp here for three days. Plus, it’s an easy opportunity to ask donors for big checks to replenish the campaign coffers.

Expect Calley to continue his scorched-earth strategy, though it’s tough to imagine what else — besides the kitchen sink — the lieutenant governor can throw at Schuette.

After multiple blunders, including the failed part-time Legislature campaign he launched a year ago on Mackinac Island, the once mild-mannered Baptist from Ionia County has used lies and gross distortions in a rather desperate attempt to defeat Schuette. If this means electing a Democrat then so be it, one insider close to term-limited Gov. Rick Snyder told me.

Calley will continue attacking the extended Schuette family’s property in the Virgin Islands, despite his campaign strategist purportedly being a resident of the U.S. territory, where you can get up to 90 percent off federal taxes.

Schuette will do his best to ignore Calley. Expect him to continue deflecting attacks by pivoting toward the general election by positioning himself as a leader without fear or favor.

Meanwhile, Democrats will be mindful of their left-flank.

Once-frontrunner Gretchen Whitmer, who has long struggled in Metro Detroit, is in the fight of her political life against insurgent, self-funding and self-declared progressive Shri Thanedar.

At the same time, neither Whitmer nor Thanedar can afford to give a lifeline to Abdul El-Sayed, a Bernie Sanders socialist whose candidacy has faded, by coming off as too cozy with big business.

The bottom line: Calley’s only chance is to go nuclear and leave nothing on the table. On the Democratic side, it remains anyone’s guess who wins.

Dennis Lennox is a political commentator and public affairs consultant.